The present invention describes a process for coating finely divided material with hydrolyzed aluminum. More specifically electroluminescent phosphors coated with hydrolyzed trimethyl aluminum have been produced which are almost completely insensitive to atmospheric moisture.
Zinc sulfide-based phosphors, typically doped with copper, may be stimulated to emit visible light by the absorption of electrical energy in a so-called electroluminescent lamp where a layer of the phosphor is sandwiched between a front transparent electrode and a back non-transparent electrode with a layer of insulating material (typically barium titanate) sandwiched between the phosphor layer and the back electrode. However, the luminescent efficiency of the phosphor degrades much more rapidly if the phosphor is exposed to a moisture-containing atmosphere than if it is exposed to a very dry atmosphere. The invention which is the subject of this disclosure was motivated by a desire to reduce the moisture sensitivity of such zinc sulfide-based phosphors. The selected approach was to form a thin yet continuous coating of hydrolyzed trimethyl aluminum (TMA) upon the surfaces of the zinc sulfide particles, thereby protecting them from the effects of atmospheric moisture.
The hydrolyzed TMA coatings are formed via chemical vapor deposition with the phosphor particles suspended within a gas-fluidized bed. Hydrolyzed TMA, presumably consisting mainly of relatively amorphous aluminum hydroxide, was selected as a coating material principally because it can be formed at relatively low temperatures by the reaction of gaseous TMA with gaseous water molecules without the use of oxygen or any other coreactant. In this way, the coatings can be formed under conditions that are least likely to modify the surface chemical composition of the relatively reactive zinc sulfide based phosphor.